The subject invention relates to steam turbines. More particularly, the invention relates to an apparatus and method for inspection of steam turbine valve seats.
Steam turbine valve seats are subject to periodic inspection for wear or damage that, if not corrected, would eventually cause significant damage to the steam turbine. In many cases, the valve seat is formed from metal, for example a wear resistant metal such as satellite, and affixed to a steel valve opening. A bond joint between the valve seat and valve opening is inspected to ensure the continued integrity of the joint. The standard inspection method for this joint utilizes penetrants. The penetrant inspection can reveal cracks or other abnormalities in the joint, but only in visible areas, for example an edge, but not in areas of the joint which are not visible. Additionally, because the valve seats are relatively inaccessible for inspection when installed in the steam turbine, the valve seats are inspected in-place in the steam turbine. There is, however, considerable risk of not detecting damage to the valve seat, valve body, or other components because of poor access and visibility of the junction between the stellite inlay and the surrounding steel of the valve seat. Further, penetrant inspection only permits an inspection of the exposed junction between the inlay material and the base material of the valve seat. Disassembly of large valve seats from valve bodies is difficult, expensive and not performed unless the seat is to be replaced with a new seat.
Frequency-based inspection, for example, ultrasonic inspection, is another possible method of inspection for valve seats. It permits the inspection of the interface between the stellite inlay and the steel of the valve seat. So it extends the inspection region to include the region that the penetrant examination cannot test. In frequency-based inspection, a probe containing one or more transducers connected to a diagnostic machine is passed over the object being tested. The transducer sends pulsewaves into the surface of the object, and an output signal is received by a receiver. The screen on the diagnostic machine will show these results in the form of amplitude and pulse readings, as well as the time it takes for the waves to reach the receiver. This data is evaluated to determine if an anomoly is present.
Frequency based inspection has not been utilized to inspect installed valve seats because of lack of access and the difficulty of accessing the stellite inlay from the end of a large valve, and/or line of sight visibility to the valve seat surface when installed. Without physical access for large valves and/or line of sight visibility, there may be noise signals present in the inspection data generated by, for example, variable lift-off of the probe from the valve seat surface and/or wobble of the probe on the valve seat surface. These noise signals may prevent accurate inspection results. Removing the valve seats for inspection presents the same risks of damage and increase in inspection time as noted above for penetrant inspection.